The "labialization" of bilabial consonants often refers to protrusion instead of a secondary articulatory feature velarization.
It is phonemically contrastive in Northwest Caucasian (e.g. Adyghe), Athabaskan, and Salishan language families, among others.
[citation needed] A few languages, including Arrernte and Mba, have contrastive labialized forms for almost all of their consonants.
In the International Phonetic Alphabet, labialization of velar consonants is indicated with a raised w modifier [ʷ] (Unicode U+02B7), as in /kʷ/.
For example, in the Athabaskan language Hupa, voiceless velar fricatives distinguish three degrees of labialization, transcribed either /x/, /x̹/, /xʷ/ or /x/, /x̜ʷ/, /xʷ/.
The extensions to the IPA has two additional symbols for degrees of rounding: Spread [ɹ͍] and open-rounded [ʒꟹ] (as in English).
[4] If precision is desired, the Abkhaz and Ubykh articulations may be transcribed with the appropriate fricative or trill raised as a diacritic: [tᵛ], [tᵝ], [tʙ], [tᵖ].
For simple labialization, Ladefoged & Maddieson (1996) resurrected an old IPA symbol, [ ̫],[5] which would be placed above a letter with a descender such as ɡ.
However, their chief example is Shona sv and zv, which they transcribe /s̫/ and /z̫/ but which actually seem to be whistled sibilants, without necessarily being labialized.